Heat exchangers are widely used to facilitate the transfer of heat from regions of higher temperature to lower temperature. Metal or carbon foam structures are commonly employed in heat exchangers, wherein such structures have a relatively large surface area and are lightweight. The vast majority of foam heat exchangers is of the open cell or open pore variety, which allow the passing of fluid through the open foam. A lower temperature fluid, whether a liquid, gas or otherwise, that is made to flow through such an open cell foam heat exchanger receives heat energy from the higher temperature solid ligaments of the foam and carries away such heat. DUOCEL, and an open cell foam manufactured by ERG of Oakland, Calif., is advertised as being useful as a heat exchanger.
However, these foam materials transport heat by conduction through solid ligaments. The thermal conductivity of the ligament material then controls the rate of heat extraction from a hot object. Because the ligaments need to be interspersed with open porosity to allow efficient heat transfer to occur to a cooling fluid, heat removal from small hot objects is problematic. The foam materials do not utilize convective heat transfer processes and can be difficult to use in small devices and other applications. So for many applications, heat pipes are the preferred method to facilitate heat transfer. A heat pipe is essentially a closed system of heat transfer in which a small amount of liquid (the “working fluid”) within a sealed and evacuated enclosure is cycled through evaporation and condensation cycles. Heat entering the enclosure at one location evaporates the working fluid at such location of higher temperature (the “evaporator region”), producing vapor that moves by convection to a location of lower temperature (the “condenser region”) within the enclosure, where it condenses back to the liquid state. Heat is absorbed by the working fluid during the evaporation process at the evaporator region. The evaporation of the working fluid increases the pressure in the evaporator region, and such pressure increase causes the vapor to flow to the condenser region. Heat is released from the working fluid during the condensation process at the condenser region. The amount of heat absorbed by the working fluid during evaporation, and the amount of heat released from the working fluid during condensation, depend on the type of working fluid used in the heat pipe.
In order for the heat transfer cycle to continue, the condensed working fluid must return to the evaporator region, where it will again be vaporized. Generally, wicks are used to permit this return, where capillary forces in the wicking structure drive the liquid back to the evaporator region. However, micro heat pipes, such as those described by Cotter (See Cotter, T. P., 1984, “Principals and Prospects of Micro Heat Pipes,” Proc. 5th Int'n Heat Pipe Conf., Tsukuba, Japan, pp. 328-335), are wickless, relying on the wicking action of their non-circular enclosure to transfer the working fluid to the evaporator region. The cyclic evaporation and condensation processes in a micro heat pipe cause a capillary pressure difference between the evaporator and condenser regions. For micro heat pipes with non-circular (preferably, cusp-shaped or triangular) cross sections, the corner regions of the heat pipe act as return channels for the working fluid, and the capillary pressure difference promotes the flow of the working fluid from the condenser region back to the evaporator region along such channels. Thus there is no need for a wick in such micro heat pipes. For heat sources with dimensions in the sub-millimeter range and smaller, like those of a microcircuit, micro heat pipes are often employed to facilitate heat transfer.
Though useful for a number of small scale applications, one and two dimensional micro heat pipes, like those claimed in U.S. Pat. Nos. 5,179,043 to Weichold, et al., 5,598,632 to Carmarda, et al., and 5,527,588 to Carmarda, et al., which are hereby incorporated by reference in their entirety herein, are limited in the art to one dimensional and two dimensional heat transfer. They are also relatively heavy, unnecessarily rigid, and expensive to produce in full arrays. Importantly, their ability to transfer heat by conduction and radiation is limited.
It would be advantageous to provide an apparatus and a method for heat exchange that effectively maximizes the benefits of convection, conduction and radiation together, resulting in heat exchange efficiency an order of magnitude superior to the present state of the art.
Furthermore, there is a need in the art for such an efficient heat exchange method to apply to a wide range of operation temperatures.
Finally, it would be advantageous to scale up the overall size of the device, while still maintaining the benefit of micro heat pipe heat transfer, for use in systems orders of magnitude larger than those presently utilizing micro heat pipes. Each of these advantages is embodied in the present invention.